1. Field of the Invention
The present invention relates to a cylindrical vibration isolating device for use in an engine mount or the like which is provided with a projection on an elastic leg to suppress a surging phenomenon due to resonance of the projection, and particularly, to the device which has the improved durability of the projection.
2. Description of the Related Art
In a cylindrical vibration isolating device used for isolating the transmission to the vehicle body of the vibration generated in an engine or the like, there is known a cylindrical bushing that a mass projection functioning as a mass (mass portion) of a dynamic damper is provided on an elastic leg so as to suppress a surging phenomenon by the resonance of the mass projection (see Japanese utility model laid-open publication No. H03-117137). The surging phenomenon is a phenomenon developing a peak of a dynamic spring constant by deformation in the direction repellent to the direction of the vibration at the time of bending resonance of the elastic leg.
FIG. 10 shows the cylindrical bushing of the prior art as above, wherein an inner cylinder 120 and an outer cylinder 130 are arranged inwardly and outwardly, and a horizontal elastic leg 140 connects the inner cylinder 120 and the outer cylinder 130. The elastic leg 140 extends substantially horizontally from the inner cylinder rightward and leftward to be connected to the outer cylinder 130. A pair of mass projections 150 projects integrally from each lateral side of the elastic leg 140 so as to suppress the bending resonance by resonating at the time of the bending resonance of the elastic leg 140.
FIGS. 11a-11b are examples for reference a mass projection is provided in a cylindrical engine mount having an elastic leg of a substantially inverted V-shape. This engine mount 1 has an inner cylinder 2 and an outer cylinder arranged in and out, and these inner cylinder 2 and outer cylinder 3 are connected by the elastic leg 4. When viewed in the axial direction of the inner cylinder 2, the elastic leg 4 is formed in a substantially inverted V-shape or a substantially arch shape having a pair of right and left leg portions. On the lateral side of each of the elastic leg portions 4 there is formed an integrally projecting mass projection 5.
In the present invention, a front view is the one viewed in the axial direction of the inner cylinder in the cylindrical vibration isolating device such as the engine mount 1 or the like. Each of the directions in the present invention is based on an illustrated state in FIGS. 11a-11b (or FIG. 1 as explained hereunder) which is the front view, wherein the upward and downward direction and the right and left direction in the drawing denote the upward and downward direction and the right and left direction of the engine mount 1, respectively. The upward and downward direction corresponds to the inputted direction (the direction shown by arrow Z) of the principal vibration to be isolated, in a state of being mounted on the vehicle body, and also corresponds to the loading direction of a static load. In addition, the term “inside” denotes the direction toward the inner cylinder 2 in the front view (in a side view or the like viewed in the direction perpendicular to the axis, the direction toward a center of longitudinal direction of the inner cylinder along the axial direction thereof).
The elastic leg 4 is connected along and to the periphery of the inner cylinder 2 by vulcanized welding or the like, and the connected portion forms an inner restrained portion 6. A tangential line in the vertical direction passing a point 6a located in an outermost position of the inner restrained portion 6 denotes an inner restraint line L1. Similarly, the elastic leg 4 is connected by vulcanization or the like to the inner wall of the outer cylinder 3, and this connected portion forms an outer restrained portion 7. When the elastic leg 4 is connected in an arch shape to the inner cylinder 2 and the outer cylinder 3, the outer restrained portion 7 are crossing points between an extension line of an outer edge portion of the elastic leg 4, which makes a larger arch shaped portion (4a, 4b) or a linear portion (4e, 4f) on an intermediate portion of the arch shaped portion (4c, 4d) of the elastic leg 4, and the outer cylinder 3. The one located on an upper side of the outer restrained portion 7 is an upper outer restrained portion 7a while the one located on a lower side thereof is a lower outer restrained portion 7b. 
A vertical line passing the lower outer restrained potion 7 denotes an outer parallel line L2. The reference character L3 denotes a tangential line passing the upper outer restrained portion 7a and connected to a point 6b of the outer periphery of the inner cylinder 2. The reference character L4 denotes a straight line parallel to the straight line L3 passing the lower outer restrained portion 7b. Also, a line connecting the upper outer restrained portion 7a and the lower outer restrained portion 7b denotes an outer restraint line L5. Further, the reference character L6 denotes a line parallel to the outer restraint line L5 and tangential to the outer periphery of the inner cylinder 2. A straight line extending parallel to the outer restraint line L5 and the parallel line L6 and provided in an intermediate position therebetween denotes an intermediate line L7. The straight line L3 is a tangential line connecting the upper outer restrained portion 7a and the contact point 6b of the outer periphery of the inner cylinder 2. The straight line L4 is a line passing the lower outer restrained portion 7b and extending parallel to the straight line L3.
The mass projection 5 is in a circular shape in a front view and is located on the inner restraint line L1 and the outer parallel line L2. Namely, the mass projection 5 is comparatively large and has a diameter greater than a space “d” between the inner restraint line L1 and the outer parallel line L2. It overlaps with the intermediate line L7, and at least a portion thereof is located inside of the intermediate line L7. Moreover, the mass projection 5 has a predetermined mass to function as a dynamic damper against the vibration of the elastic leg 4 and is set such that it develops resonance in a resonance frequency range of the elastic leg 4.
FIG. 4 is a graph showing dynamic characteristics against an excitation frequency. The curve shown in a phantom line denotes a dynamic spring constant curve of a comparative example without the mass projection 5. By the bending resonance of the elastic leg 4, it reaches a peak “a” of large dynamic spring constant in the vicinity of 900 Hz. At this peak “a” there is a decrease in vibration isolating effect.
A curve in a solid line denotes a dynamic spring constant curve of the example with the mass projection for reference as above and has peaks “b” and “c” of comparatively small dynamic spring constant in front of and behind the peak “a”. The peak “b” is located for example around 750 Hz while the peak “c” is located for example in the vicinity of 1000 Hz. Accordingly, by provision of the mass projection 5, the dynamic spring peak is decreased from “a” to “c” in a range of 700˜1000 HZ corresponding to a frequency of gear noises of a transmission, so as to provide a noticeable improvement in the dynamic spring by the difference “Dc” whereby it is possible to absorb the vibration due to the gear noises.
Incidentally, in the prior art shown in FIG. 10, the elastic leg 140 is provided substantially horizontal and has a substantially horizontal twin beam shape each end of which is restrained by an inner restraint line L1 and an outer restraint line L5. Therefore, since the mass projection 150 is located in a center of a shear deformation region of comparatively long span, the strain against a base portion of the mass projection 150 is decreased in comparison with the strain by the compression deformation, even if the elastic leg 140 generates the bending vibration. Thus, the crack or the like does not occur around the mass projection 150 at the vibration frequency of generally requested level, so that it is possible to obtain the sufficient durability with respect to the mass projection 150.
In the meantime, when this mass projection is provided in the engine mount 1 having the elastic leg 4 of a substantially inverted V-shape as in the example for reference in FIG. 11a, there are cases where the crack or the like in the base portion of the mass projection 5 occurs at the comparatively small vibration frequency (the order of tens of thousands) of generally requested level thereby decreasing the durability, as shown in FIG. 11b. Therefore, it is practically difficult to automatically provide the mass projection 5 on the substantially inverted V-shaped elastic leg 4. It has become clear from the results of the research that the crack or the like is caused by overlapping of the mass projection 5 with each of the inner restraint line L1 and the outer parallel line L2, especially, with the inner restraint line L1 and by overlapping of the mass projection 5 with the intermediate line L7 so as to have the center of the projection located inside thereof.
Namely, it is generally thought that the mass projection 5 is arranged in the central area of the lateral side of the elastic leg 4 where there is a comparatively large free space for easy arrangement. However, the intermediate line L7 is located in the center of the lateral side, and the space “d” between the inner restraint line L1 and the outer parallel line L2 is small in the substantially inverted V-shaped elastic leg 4 so as to have these lines located comparatively close to each other. Therefore, when the mass projection 5 is arranged in the center of the elastic leg 4, it is easy to overlap with the inner restraint line L1, the outer parallel line L2 and the intermediate line L7. In addition, since the mass projection 5 having a greater diameter than the space “d” is provided, it is difficult to avoid overlapping with these restraint lines.
However, the inside of the inner restraint line L1 is a compression region A comprised mainly of compression and tension deformation, the area between the inner restraint line L1 and the outer parallel line L2 is a shear region B comprised mainly of shear deformation, and the outside of the outer parallel line L2 is a compression and shear region C where the compression deformation and the shear deformation are mixed. Then, the compression region A is the greatest spring constant region, the compression and shear region C is the second greatest spring constant region, and the shear region B is the smallest spring constant region. Accordingly, the greatest region in rubber leg deformation amount is the shear region B. Therefore, the region located on the inner restraint line L1 and the outer parallel line L2 is the region developing the greatest difference in the deformation amount so that when the mass projection 5 overlaps with these lines, the high strain is generated in the base portion of the mass projection 5. Then, as shown in FIG. 11b, which is a partial enlarged side view of the mass projection 5 of FIG. 11a, the crack 9 occurs in the base portion 8 of the mass projection 5 thereby decreasing the durability.
FIGS. 12a-b are views for explaining the vibration of the elastic leg 4 in the engine mount. FIG. 12a shows an upwardly moved state of the inner cylinder 2 and the elastic leg 4 is under tension by the pulling action of the inner cylinder 2. At that time, while the mass projection 5 is pulled by the inner cylinder 2 so as to move from a position shown in a phantom line to a position shown in a solid line, it is located between the inner restrained portion 6 and the outer restrained portion 7, so that the high strain due to the tension deformation is developed between the lower outer restrained portion 7b and the base portion 8 of the mass projection 5.
FIG. 12b shows a downwardly moved state of the inner cylinder 2 and the elastic leg 4 is compressed downward by the inner cylinder 2. At that time, since the mass projection 5 is pressed by the inner cylinder 2 so as to move from a position shown in a phantom line to a position shown in a solid line, the high strain is caused in the base portion of the mass projection 5 by the compression deformation between the inner cylinder 2 and the mass projection 5.
In the case where such upward and downward vibrations are continuously repeated, for example, in the case of having applied the vibrations which cause the upward displacement of 5.9 mm and the downward displacement of 11.5 mm, the crack occurred in the base portion 8 of the mass projection 5 at about twenty thousands of the vibration frequencies. Therefore, the present invention has its object to improve the durability by preventing the occurrence of such crack up to greater vibration frequencies.